Expressions

Forge Json includes a built-in expression engine that lets you write dynamic formulas using the {{expression}} syntax. Expressions are used across multiple features to transform, filter, and compute values.

Where expressions are used

Feature	What expressions do	Example
Find & Replace	Dynamic replacement values using the `old` variable	`{{old.toUpperCase()}}`
Tabular Operations	Filter rows, compute new columns, conditional updates	`{{age >= 18}}`
Generate CSV	Map JSON fields to CSV columns	`{{stringify(address)}}`
Compute Fields	Create derived fields from existing data	`{{price}} * {{quantity}}`

Quick examples

String transformation (Find & Replace)


Search:    hello
Replace:   {{old.toUpperCase()}}
Result:    HELLO

Row filtering (Tabular Operations)


Condition: {{age >= 18}}

Given [{"name": "Alice", "age": 25}, {"name": "Bob", "age": 12}], this keeps only Alice’s row.

Computed field


Expression: {{firstName}} {{lastName}}
Target:     fullName

Given {"firstName": "Jane", "lastName": "Doe"}, produces {"firstName": "Jane", "lastName": "Doe", "fullName": "Jane Doe"}.

Math


Expression: {{price * quantity}}

Given {"price": 10, "quantity": 5}, returns 50.

How it works

The expression engine uses JSEP (JavaScript Expression Parser) to parse expressions into an abstract syntax tree (AST), then evaluates them safely — no eval() is ever used.

Expressions are read-only — they can access and compute values but cannot modify the original data. Transformations are always applied as new values.

Security

The expression engine enforces strict safety rules:

AST-based evaluation — no eval() or new Function()
Maximum recursion depth of 20 levels
Forbidden property access — __proto__, prototype, and constructor are blocked
No computed property access — obj[key] syntax is not allowed; use dot notation instead
Whitelisted functions only — only approved Math, Date, and utility functions are available

Next steps

Template Syntax — how {{...}} templates work
Operators — arithmetic, comparison, and logical operators
Built-in Functions — Math, Date, and utility functions
String Methods — methods available on the old variable
Field Access — how to reference fields in your data
Usage Contexts — how expressions differ across features